Broadband signals are used in digital communication systems and navigation systems. In particular, in the satellite navigation systems of GPS (NAVASTAR) and GLONASS (GLN), a receiver processes a great number of broadband signals, each of which is radiated by a corresponding satellite. The location of the receiver and the velocity of its movement are determined as a result of processing. Broadband signals in communication systems are used to increase noise-immunity, to raise secrecy of working, and to separate communication channels in multi-channel radio links.
The typical method of generating broadband signals is based on the use of pseudo-random codes (PR-codes), which modulate the carrier. Typically, the transmitter generates a finite sequence of +1 and -1 bits, or "chips", which follow the pattern of a PR-code of finite length (called the code duration). This signal has a broad frequency spectrum due to the near random pattern of the code. The transmitter continually repeats the generation of this finite sequence, and may modulate the repeating sequence with an information signal. Such modulation may be done by multiplying a group of whole sequences by +1 or -1, as dictated by the information signal. In doing so, the relatively narrow frequency spectrum of the information signal is expanded, or "spread", out to that of the repeating PR-code sequence. The modulated broadband signal is then used to modulate a carrier signal for transmission (i.e., up-converted).
If the repeating pseudo-random signal is not modulated by an information signal, it may nonetheless be useful to a receiver. For example, each GPS satellite transmits two repeating pseudo-random signals in two different frequency bands (L1 and L2), respectively, at precise and synchronized times. Comparison of the transmission delays of the two signals enables the receiver to determine the effects of the ionosphere on the signal transmission. The comparison could be done with the L1 signal being modulated by an information signal, and with the L2 signal not being modulated by an information signal.
In a receiver, the broadband signal is down-converted and then compressed, or "de-spread", by correlating the received signal with a locally generated version of the PR-code (which is often called the reference PR-code). The local version may be an identical replica of the PR-code used by the transmitter, or it may be a derivative of the PR-code, such as a strobed version. If a narrow-band interference signal is received at the input of such receiver along with the transmitter's signal, the interference signal will be suppressed because its frequency spectrum is much less than that of the transmitter's signal. The degree of suppression depends on the base of the PR-code, which in turn is determined by the ratio of the code duration to the duration of one chip element (bit) of the code.
In many cases such suppression turns out to be insufficient. In the systems of GPS and GLN, the signal power of a useful signal at a receiver input is usually considerably less than the power level of noise signals received by the receiver input and generated by the receiver's components, and the power level of a narrow-band interference signal often considerably exceeds the noise power level. As a result, even after the correlation processing, the effect of the narrow-band interference remains too strong to be able to properly demodulate the transmitter's signal.
The suppression of the narrow-band interference may be considerably increased by using the compensation method. According to this method, two signal-processing paths and a compensator are created in a receiver. The first processing path extracts the interference signal from the received input signal, using the following two characteristics which distinguish the interference signal from the useful signal and noise: the narrow bandwidth frequency of the interference signal and its higher power. A locally generated copy of the interference is then generated at the output of the first processing path. This copy is then subtracted from the received input signal by the compensator, and the resulting difference signal is provided to the second signal processing path. The signal provided to the second path has thereby been "compensated" by the subtraction of the interference copy signal, and the second path can then proceed with the usual steps of compressing and demodulating the transmitter's signal. In the difference signal generated by the compensator, the interference signal will be suppressed, and the degree of the suppression will depend upon how close the interference copy is to the original interference signal provided to the compensator. The steps of correlation processing, compressing the useful signal, and suppressing the uncompensated interference residue are realized after the above-described compensation.
There are different known methods of separating a narrow-band interference signal from the broadband useful signal and noise so that a copy of the interference signal can be generated for the compensator. These methods are briefly described below.
One of the known methods is based on application of a band limiter, as described by J. J. Spilker, et al., "Interference Effects and Mitigation Techniques," Global Positioning System: Theory and Applications, Volume I (1996). The limiter is captured by a strong narrow-band interference signal (whose power considerably exceeds the total power of the useful signal and noise). A signal in which the interference essentially dominates is obtained after the limiter. The output signal of the limiter is considered to be a copy of the interference signal, and is subtracted from a copy of the input signal which has been passed through an amplitude regulator. As a result, interference suppression is observed with comparatively insignificant distortion of the broadband useful signal.
In another method described in U.S. Pat. No. 5,268,927, the copy of the narrow-band interference signal is obtained by means of an adaptive transversal filter. This method provides better interference suppression than the previously-described method.
This method described in U.S. Pat. No. 5,268,927 is based on the premise that the respective correlation intervals of the useful signal and of the noise in the receiver band are much less than the correlation interval of a narrow-band interference signal. Therefore, it is possible to sample the input signal at a sampling frequency which is selected to provide a weak correlation to the useful signal and noise but strongly correlated to the interference signal. This sampling enables one to predict the parameters (e.g., weight functions) of a programmable transversal filter (PTF) which can separate the interference signal from the useful signal (and noise) and generate at its output a copy of the interference signal, which in turn can be used by the compensator. The performance characteristics of the compensator are determined by the chosen algorithm of adapting the parameters of the PTF to track the changes in the interference signal. Both the algorithm and the resulting performance essentially depend upon the practical realization of the algorithm, the PTF, and the compensator on a concrete calculating device (e.g., digital signal processor). In practice, the adjustment algorithm for the adaptive programmable transversal filter turns out to be extremely complex and greatly taxes the processor of the digital receiver.
Another known method is based on the results from the Markov theory of the non-linear synthesis, and is described by G. I. Tuzov, "Statistical Theory of Reception of Compound Signals," Soviet Radio, Moscow, 1977. According to these results, it is necessary to separate a narrow-band interference having a constant amplitude from the input signal by means of a phase-lock loop (PLL) system which is locked onto the interference signal. If the amplitude of the interference signal is unknown, it is also necessary to determine the amplitude by another device, in which synchronous detection of the input signal with a reference signal generated by the PLL system is employed. The thus-found amplitude is filtered and then multiplied by the PLL reference signal, forming a copy of the interference signal. These various processing functions lead to a relatively complex receiver. If it can be assumed that the power level of the interference signal exceeds the combined power level of the useful signal and all noise sources, then some of the above processing functions can be implemented by simplified components to provide a less complex receiver. However in order to use the device in real conditions, one finds that it is necessary to match the amplitude-frequency response of the PLL system with the frequency spectrum of the interference signal, which is unknown beforehand. Unfortunately, this matching process requires another PLL system tuned on the mean frequency of the interference signal. Even when the matching process is undertaken with the additional PLL system, the degree of interference compensation degree will be comparatively small, since it is difficult to achieve good coincidence of the interference copy with its original in such circuit.
In the typical case, the result of sending the input signal through a narrow-band interference compensator is equivalent to the result of sending the input signal through a rejection filter which cuts out a frequency band of the interference signal. It is natural that the corresponding section of the signal spectrum is cut out together with the interference, which causes distortions in the useful signal. However, if the frequency spectrum of the input signal is significantly broader than that of the interference signal, the distortions are not large and, in any case, are considerably less than those seen when no compensation is used.
In order to realize effective compensation it is necessary to tune this equivalent rejection filter correctly. The adaptive transversal filter automatically realizes such tuning by continuously comparing its output to the interference signals and periodically adjusting its parameters (e.g., weights) to obtain the best tunning.
A rejection filter which is tuned up only according to the central frequency, which must coincide with the mean interference frequency, is used in simpler compensators (see, for example, J. J. Spilker, et al., "Interference Effects and Mitigation Techniques," Global Positioning System: Theory and Applications, Volume I (1996)). Different kinds of spectrum analyzers are used to estimate the mean frequency.
There is the well-known spectrum analyzer which generates and processes two mutually orthogonal signals I and Q. These signals are obtained by correlating the input signal with two reference harmonic signals sin .omega..sub.0 t and cos .omega..sub.0 t, which are orthogonal to one another. An power signal Z=(I.sup.2 +Q.sup.2) is generated and is proportional to power of the input signal in a small frequency band around the reference frequency .omega..sub.0. The bandwidth of this small frequency band is inversely proportional to the accumulation time T.sub.A in the correlators. T.sub.A may, of course, be adjusted. If the value T.sub.A is sufficiently large, it is possible to consider that signal Z is proportional to spectral density of the input signal at the frequency .omega..sub.0. The frequency value of .omega..sub.0 may then be swept through a desired frequency range to find the spectral density of the input signal. A narrow-band interference signal is detected in such an analyzer when a large, noticeable spike (or ejection) is seen. The frequency center of the spike provides an estimation of the mean frequency of the interference signal.
The quality of an interference suppresser is determined by many factors, which often turn out to be conflicting. The more the suppresser cuts out an interference and the less it distorts a useful signal, the better it will be. These factors are improved under the most absolute matching of the suppresser characteristics with the interference signal. However, the absolute matching, as a rule, is achieved either at the expense of complication and increase in the price of a suppresser, or at the cost of limiting the device application possibilities to only one kind of interference. Therefore, a designer is always faced with having to make compromises.
The purpose of this invention is to provide methods of narrow-band interference suppression which achieve good suppression of a wide class of narrow-band interferences with a comparatively small complication of hardware and acceptable increase of processor loading.